Add a comet trail animation

The frame glides from waypoint to waypoint and drops a screenshot of
itself every frame-width or so. The stills stay where they were taken
and fade over a few seconds, so the live video reads as the head of a
comet whose tail is its own recent past.

The steering caps how fast the heading may change: an instant turn would
lay the stills out in straight dotted lines, and it is the curve that
makes a tail read as one. Drops are spaced by distance travelled rather
than by a timer, so they stay even however the frame is turning.

Since the capture is a crop of the tab, a still lying under the frame
would be photographed into the next one. A drop waits until the frame is
clear of the tail, keeping the invariant corners already relies on.

Also picks up the FRAME_SCALE, BOUNCE_SPEED and CORNER_INTERVAL_SECONDS
tuning sitting in the tree.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Tg15tLAuT6NrCE1ZxKVgDB

server/index.js

@@ -18,6 +18,11 @@ import {
   DRAG_FRAME_SCALE,
   DRAG_MIN_SCALE,
   DRAG_MAX_SCALE,
+  TRAIL_SPEED,
+  TRAIL_TURN_RATE,
+  TRAIL_SPACING,
+  TRAIL_LENGTH,
+  TRAIL_FADE_SECONDS,
   BACKGROUNDS,
   ANIMATIONS,
   AUTO_ANIMATION,
@@ -51,6 +56,11 @@ app.get('/config.js', (_req, res) => {
       `export const DRAG_FRAME_SCALE = ${DRAG_FRAME_SCALE};\n` +
       `export const DRAG_MIN_SCALE = ${DRAG_MIN_SCALE};\n` +
       `export const DRAG_MAX_SCALE = ${DRAG_MAX_SCALE};\n` +
+      `export const TRAIL_SPEED = ${TRAIL_SPEED};\n` +
+      `export const TRAIL_TURN_RATE = ${TRAIL_TURN_RATE};\n` +
+      `export const TRAIL_SPACING = ${TRAIL_SPACING};\n` +
+      `export const TRAIL_LENGTH = ${TRAIL_LENGTH};\n` +
+      `export const TRAIL_FADE_SECONDS = ${TRAIL_FADE_SECONDS};\n` +
       `export const BACKGROUNDS = ${JSON.stringify(BACKGROUNDS)};\n` +
       `export const ANIMATIONS = ${JSON.stringify(ANIMATIONS)};\n` +
       `export const AUTO_ANIMATION = ${JSON.stringify(AUTO_ANIMATION)};\n`,